polonium

polonium

polonium, radioactive chemical element; symbol Po; at. no. 84; mass no. of most stable isotope 209; m.p. 254°C;; b.p. 962°C;; sp. gr. about 9.4; valence +2 or +4. Polonium is an extremely rare element found in uranium ores (about 0.1 gram per ton). A product of radium decay, it is sometimes called radium F. In its physical and chemical properties it resembles tellurium (the element above it in Group 16 of the periodic table) and bismuth. Polonium has 34 isotopes, more than any other element. All of these isotopes are radioactive. The most stable, polonium-209, has a half-life of about 103 years. Polonium-208 (half-life about 3 years) is the only other polonium isotope with a half-life over one year. Although these two isotopes can be prepared in small quantities in a particle accelerator, they are very expensive to produce. All other polonium isotopes are short-lived except polonium-210 (half-life about 138 days), which is the most commonly used isotope. It is prepared by bombarding bismuth with neutrons in a nuclear reactor. It is a highly radioactive material. A milligram of polonium-210 emits as much alpha radiation as about 5 grams of radium, and enough gamma radiation to cause a blue glow in the air around it. It can be used as a heat source, since most of the energy of the alpha radiation is absorbed as heat within the polonium and its container. Polonium has found use in small portable radiation sources and in the control of static electricity. However, it is an extremely toxic substance and must be handled with great care. Polonium was the first element to be discovered because of its radioactivity; it was discovered in pitchblende in 1898 by Marie Curie and named for her native country, Poland.

Characteristics

Isotopes

Polonium has 25 known isotopes, all of which are radioactive. They have atomic masses that range from 194u to 218u. 210Po (half-life 138.376 days) is the most widely available. 209Po (half-life 103 years) and 208Po (half-life 2.9 years) can be made through the alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron.210Po
210Po is an alpha emitter that has a half-life of 138.376 days; it decays directly to its daughter isotope206Pb. A milligram of 210Po emits about as many alpha particles per second as 4.5 grams of 226Ra. A few curies (1 curie equals 37 gigabecquerels) of 210Po emit a blue glow which is caused by excitation of surrounding air. A single gram of 210Po generates 140 watts of power. Because it emits many alpha particles, which are stopped within a very short distance in dense media and release their energy, 210Po has been used as a lightweight heat source to power thermoelectric cells in artificial satellites; for instance, 210Po heat source was also used in each of the Lunokhod rovers deployed on the surface of the Moon, to keep their internal components warm during the lunar nights. Some anti-static brushes contain up to of 210Po as a source of charged particles for neutralizing static electricity in materials like photographic film. 210Po was also used as a murder weapon in the Alexander Litvinenko poisoning.

The majority of the time 210Po decays by emission of an alpha particle only, not by emission of an alpha particle and a gamma ray. About one in 100,000 alpha emissions causes an excitation in the nucleus which then results in the emission of a gamma ray. This low gamma ray production rate (and the short range of alpha particles) makes it difficult to find and identify this isotope. Rather than gamma ray spectroscopy, alpha spectroscopy is the best method of measuring this isotope.

Solid state form

The alpha form of solid polonium has a simple cubic crystal structure with an edge length of 335.2 picometres.

The beta form of polonium is rhombohedral; it has been reported in the chemical literature, along with the alpha form, several times. A picture of it is present on the web.

Two papers report X-ray diffraction experiments on polonium metal. The first report of the crystal structure of polonium was done using electron diffraction.

Chemistry

The chemistry of polonium is similar to that of tellurium and bismuth. Polonium dissolves readily in dilute acids, but is only slightly soluble in alkalis. The hydrogen compound is liquid at room temperature (M.P. -36.1°C to B.P. 35.3°C). Halides of the structure PoX2, PoX4 and PoX6 are known. The two oxides PoO2 and PoO3 are the products of oxidation of polonium.

210Po (in common with 238Pu) has the ability to become airborne with ease: if a sample is heated in air to 328 K (55°C, 131°F), 50% of it is vaporized in 45 hours, even though the melting point of polonium is 527 K (254°C, 489°F) and its boiling point is 1235 K (962°C, 1763°F).
More than one hypothesis exists for how polonium does this; one suggestion is that small clusters of polonium atoms are spalled off by the alpha decay.

It has been reported that some microbes can methylate polonium by the action of methylcobalamin.This is similar to the way in which mercury, selenium and tellurium are methylated in living things to create organometallic compounds. As a result when considering the biochemistry of polonium one should consider the possibility that the polonium will follow the same biochemical pathways as selenium and tellurium.

Compounds

History

Also tentatively called "Radium F", polonium was discovered by Marie Skłodowska-Curie and her husband Pierre Curie in 1898 and was later named after Marie Curie's native land of Poland (Latin: Polonia), and not for the Hamlet character, Polonius.
Poland at the time was under Russian, Prussian, and Austrian partition, and did not exist as an independent country. It was Curie's hope that naming the element after her native land would publicize its lack of independence. Polonium may be the first element named to highlight a political controversy.

This element was the first one discovered by the Curies while they were investigating the cause of pitchblenderadioactivity. The pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than both the uranium and thorium put together. This spurred the Curies on to find additional radioactive elements. The Curies first separated out polonium from the pitchblende, and then within a few years, also isolated radium.

Detection

Gamma counting

By means of radiometric methods such as gamma spectroscopy (or a method using a chemical separation followed by an activity measurement with a non-energy-dispersive counter), it is possible to measure the concentrations of radioisotopes and to distinguish one from another. In practice, background noise would be present and depending on the detector, the line width would be larger which would make it harder to identify and measure the isotope. In biological/medical work it is common to use the natural 40K present in all tissues/body fluids as a check of the equipment and as an internal standard.

Alpha counting

The best way to test for (and measure) many alpha emitters is to use alpha-particle spectroscopy as it is common to place a drop of the test solution on a metal disk which is then dried out to give a uniform coating on the disk. This is then used as the test sample. If the thickness of the layer formed on the disk is too thick then the lines of the spectrum are broadened, this is because some of the energy of the alpha particles is lost during their movement through the layer of active material. An alternative method is to use internal liquid scintillation where the sample is mixed with a scintillation cocktail. When the light emitted is then counted, some machines will record the amount of light energy per radioactive decay event. Due to the imperfections of the liquid scintillation method (such as a failure for all the photons to be detected, cloudy or coloured samples can be difficult to count) and the fact that random quenching can reduce the number of photons generated per radioactive decay it is possible to get a broadening of the alpha spectra obtained through liquid scintillation. It is likely that these liquid scintillation spectra will be subject to a Gaussian broadening rather than the distortion exhibited when the layer of active material on a disk is too thick.

A third energy dispersive method for counting alpha particles is to use a semiconductor detector.

From left to right the peaks are due to 209Po, 210Po, 239Pu and 241Am. The fact that isotopes such as 239Pu and 241Am have more than one alpha line indicates that the nucleus has the ability to be in different discrete energy levels (like a molecule can).

Occurrence and production

Polonium is a very rare element in nature because of the short half-life of all its isotopes. It is found in uranium ores at about 100 micrograms per metric ton (1 part in 1010), which is approximately 0.2% of the abundance of radium. The amounts in the Earth's crust are not harmful. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers.

Neutron capture

Synthesis by (n,gamma) reaction

In 1934 an experiment showed that when natural 209Bi is bombarded with neutrons, 210Bi is created, which then decays to 210Po via β decay. The final purification is done pyrochemically followed by liquid-liquid extraction techniques. Polonium may now be made in milligram amounts in this procedure which uses high neutron fluxes found in nuclear reactors. Only about 100 grams are produced each year, practically all of it in Russia, making polonium exceedingly rare.

Proton capture

Synthesis by (p, n) and (p,2n) reactions

It has been found that the longer-lived isotopes of polonium can be formed by proton bombardment of bismuth using a cyclotron. Other more neutron rich isotopes can be formed by the irradiation of platinum with carbon nuclei.

Devices that eliminate static charges in textile mills and other places. However, beta particle sources are more commonly used and are less dangerous. A non-radioactive alternative is to use a high-voltage DC power supply to ionise air positively or negatively as required.

Toxicity

Overview

By mass, polonium-210 is around 250,000 times more toxic than hydrogen cyanide (the actual LD50 for 210Po is about 1 microgram for an 80 kg person (see below) compared with about 250 milligrams for hydrogen cyanide). The main hazard is its intense radioactivity (as an alpha emitter), which makes it very difficult to handle safely: one gram of Po will self-heat to a temperature of around . Even in microgram amounts, handling 210Po is extremely dangerous, requiring specialized equipment and strict handling procedures. Alpha particles emitted by polonium will damage organic tissue easily if polonium is ingested, inhaled, or absorbed (though they do not penetrate the epidermis and hence are not hazardous if the polonium is outside the body).

Acute effects

The median lethal dose (LD50) for acute radiation exposure is generally about 4.5 Sv. The committed effective dose equivalent210Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if inhaled. Since 210Po has an activity of 166 TBq (4486.5 Ci) per gram (1 gram produces 166×1012 decays per second), a fatal 4.5 Sv (J/kg) dose can be caused by ingesting 8.8 MBq (238 microcuries), about 50 nanograms (ng), or inhaling 1.8 MBq (48 microcuries), about 10 ng. One gram of 210Po could thus in theory poison 20 million people of whom 10 million would die. The actual toxicity of 210Po is lower than these estimates, because radiation exposure that is spread out over several weeks (the biological half-life of polonium in humans is 30 to 50 days) is somewhat less damaging than an instantaneous dose. It has been estimated that a median lethal dose of 210Po is 0.015 GBq (0.4 millicuries), or 0.089 micrograms, still an extremely small amount.

Long term (chronic) effects

In addition to the acute effects, radiation exposure (both internal and external) carries a long-term risk of death from cancer of 5–10% per Sv. The general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes 214Po and 218Po are thought to cause the majority of the estimated 15,000-22,000 lung cancer deaths in the US every year that have been attributed to indoor radon. Tobacco smoking causes additional exposure to Po.

Regulatory exposure limits

The maximum allowable body burden for ingested 210Po is only 1.1 kBq (30 nCi), which is equivalent to a particle massing only 6.8 picograms. The maximum permissible workplace concentration of airborne 210Po is about 10 Bq/m3 (3 × 10−10 µCi/cm³). The target organs for polonium in humans are the spleen and liver. As the spleen (150 g) and the liver (1.3 to 3 kg) are much smaller than the rest of the body, if the polonium is concentrated in these vital organs, it is a greater threat to life than the dose which would be suffered (on average) by the whole body if it were spread evenly throughout the body, in the same way as caesium or tritium (as T2O).

210Po is widely used in industry, and readily available with little regulation or restriction. In the US, a tracking system run by the Nuclear Regulatory Commission will be implemented in 2007 to register purchases of more than of polonium 210 (enough to make up 5,000 lethal doses). The IAEA "is said to be considering tighter regulations... There is talk that it might tighten the polonium reporting requirement by a factor of 10, to ."

It has also been suggested that Irène Joliot-Curie was the first person ever to die from the radiation effects of polonium (due to a single intake) in 1956. She was accidentally exposed to polonium in 1946 when a sealed capsule of the element exploded on her laboratory bench. A decade later, on 17 March1956, she died in Paris from leukemia which may or may not have been caused by that exposure.

According to the book The Bomb in the Basement, several death cases in Israel during 1957-1969 were caused by 210Po. A leak was discovered at a Weizmann Institute laboratory in 1957. Traces of 210Po were found on the hands of Prof. Dror Sadeh, a physicist who researched radioactive materials. Medical tests indicated no harm, but the tests did not include bone marrow. Sadeh died from cancer. One of his students died of leukemia, and two colleagues died after a few years, both from cancer. The issue was investigated secretly, and there was never any formal admission that a connection between the leak and the deaths had existed.

Treatment

It has been suggested that chelation agents such as British Anti-Lewisite (dimercaprol) can be used to decontaminate humans. In one experiment, rats were given a fatal dose of 1.45 MBq/kg (8.7 ng/kg) of 210Po;
all untreated rats were dead after 44 days, but 90% of the rats treated with the chelation agent
HOEtTTC remained alive after 5 months.

Commercial products containing polonium

No credible nuclear authority has asserted that a commercial product was a likely source for the poisoning of Litvinenko. However, as Prof. Peter D. Zimmerman
says, "Polonium 210 is surprisingly common. ...Polonium sources with about 10 percent of a lethal dose are readily available — even in a product sold on Amazon.com."

Potentially lethal amounts of polonium are present in anti-static brushes sold to photographers. Many of the devices are available by mail order.
General Electric markets a static eliminator module with 500 microcuries (20 MBq), roughly 2.5 times the lethal dose of 210Po if 100%-ingested, for US $71; Staticmaster sells replacement units with the same amount (500 mCi) of 210Po for $36. In USA, the devices with no more than 500 mCi of (sealed) 210Po per unit can be bought in any amount under a "general license" which means that a buyer needn't be registered by any authorities: the general license "is effective without the filing of an application with the Commission or the issuance of a licensing document to a particular person."

If these sources were used to collect the amount of polonium likely used in the poisoning—and one could devise a method of separating the polonium from its protective casing—it would take 10-100 modules for price of US $360 to $7,100. That such a thing could be done is extremely difficult according to the manufacturers and would be highly dangerous to anyone attempting to do so without some special equipment like a glovebox.

Sometimes sources of polonium used in industry are stolen or lost. According to the National Regulatory Commission, there were registered at least 8 cases of loss of control of potentially lethal polonium sources in the USA during 2006.
Tiny amounts of such radioisotopes are sometimes used in the laboratory and for teaching purposes — typically of the order of 4–40 kBq (0.1–1.0 muCi), in the form of sealed sources, with the Po deposited on a substrate or in a resin or polymer matrix—are often exempt from licensing by NRC and similar authorities as they are not considered hazardous. Small amounts of 210Po are available to the public in the United States by mail order from a company called United Nuclear as 'needle sources' for laboratory experimentation. It would require about 15,000 210Po of these sources at a total cost of about $1 million to obtain a toxic quantity of Polonium. They typically sell between 4 and 8 sources per year.

According to some estimates, the cost of the quantity of pure Polonium-210 used to kill Litvinenko would be around £20 million (US $39 million). However, this estimation is based on retail prices of commercially available demonstration radiation sources with very small activities and cannot be considered as reasonable.

Tobacco

The presence of polonium in tobacco smoke has been known since the early 1960s,. Some of the world's biggest tobacco firms researched ways to remove the substance – to no avail – over a 40-year period but never published the results.

Radioactive Polonium-210 contained in phosphate fertilizers is adsorbed by the roots of plants (such as tobacco) and stored in its tissues. Tobacco derived from plants fertilized by rock phosphates contains Polonium-210 which emits alpha radiation estimated to cause about 11,700 lung cancer deaths each year worldwide.